1. Field of Invention
The present invention relates to a coincidence event identification technology, and in particular, to a radiation signal processing method and a radiation signal processing system which combine trigger signals and time marks.
2. Related Art
Positron emission tomography (PET) uses an isotopic drug having a small amount of radiation to perform biomedical detection. After glucose having a radioactive medicament enters an organism through intravenous injection, a large amount of the medicament is absorbed by malignant cells. When a positron in a decaying process and an electron in a cell collide to mutually counteract and destroy each other to generate an annihilation effect, the masses of the positron and the electron disappear, and two γ-rays are released in opposite directions and form an angle of 180°. The energy of each γ-ray is 511 keV. By detecting these paired γ-rays, a PET scanner reestablishes a distribution situation of a positron drug in a tissue or an organ to obtain a desired image. Therefore, how to accurately identify the paired γ-rays in real time is a problem currently concerned by those in the industry.
The existing technologies for identifying paired γ-rays may be classified into three types: (1) a time mark method, (2) a trigger signal AND logic method, and a hybrid method. In the time mark method, an event coincidence detection system circuit sorts time marks sent by all detectors in a fixed time cycle, and computes time mark difference values between the paired detectors to determine whether a time mark difference value of a detector module pair is within a preset time coincidence window. If a time mark difference value is smaller than a numerical value of the time coincidence window, it is determined that a coincidence event occurs to this pair. This method can provide accurate time resolution, but the complexity in implementation increases as the lengths of time mark digital values increase, which causes poor real time performance of the detection system circuit.
In the trigger signal AND logic method, trigger signals output by all possibly paired detectors are detected by using an “AND” logic gate to see whether the trigger signals are simultaneously generated. An approach is to find all possible detector pair combinations in advance, and trigger signals of two detectors in each combination pass through an AND logic gate. Only when the two paired trigger signals are both at high levels, the AND logic gate produces a high level to determine that a coincidence event occurs to this pair. This method is implemented in real time and is fast and simple, which saves the sorting of paired events in the time mark method, thereby greatly reducing the system complexity and making it easy to implement on a hardware circuit. However, the magnitude of the time coincidence window is decided by pulse widths of the trigger signals, so that it is not suitable for real time changing and adjustment, and the trigger signals are easily interfered and influenced by noises of circuits and components.
Finally, in the hybrid method, preliminary filtering is performed by the conventional trigger signal AND logic method, and for paired modules after the filtering, the time mark method is used to finally identify a coincidence event. An approach is to firstly synchronize an original trigger signal output by each detector module with a system main clock to obtain a synchronized trigger signal. All possible detector module pair combinations are found, and synchronized trigger signals of two paired modules in each combination pass through an AND logic gate. When the two paired synchronized trigger signals are both at high levels, the AND logic gate produces a high level. Accordingly, it is preliminarily determined that a coincidence event possibly occurs to this pair. For preliminarily selected candidate paired modules of the coincidence event, coincidence event identification is performed by using the time mark method in a second stage. Time marks output by the selected paired modules are analyzed in detail to finally decide whether the coincidence event truly occurs.
To sum up, the hybrid technology for identifying paired γ-rays in the prior art is to perform the preliminary filtering by the conventional trigger signal AND logic method, and use the time mark method to finally identify the coincidence event after the filtering. Though the real time performance of the conventional trigger signal AND logic method and the accuracy of the conventional time mark method are achieved at the same time, each detector in the system is matched with a set of a pulse discrimination circuit and a time-to-digital converter (TDC), which need a large quantity of chips and memories, thereby causing a waste of resources and an increased cost. Moreover, though this technology avoids the procedure of sustained and complicated comparisons of all time mark signals in the conventional time mark method, the efficiency of the chips still needs to be consumed to perform a large quantity of subtraction computation and comparison procedures.